Method and apparatus in a wireless communication system

ABSTRACT

A method and apparatus for determining when to switch between buffer estimation states for a fixed frame rate services session at a terminal. In the event that the state is passive, a network node determines a first time between two received Radio Link Control Service Data Units (RLC SDU). If the first time is lower than a first time threshold, then the state is changed to proactive. When the state is proactive, the network node determines that two consecutive padding transport blocks have been received and further determines that a second time between two padding transport blocks is greater than a second time threshold. If so then the state is changed to passive.

This application is a 371 of International Application No.PCT/EP2013/071591, filed Oct. 16, 2013, the disclosure of which is fullyincorporated herein by reference.

TECHNICAL FIELD

The present application relates to a method and apparatus in a wirelesscommunication system, and in particular to a method of determining whento switch between buffer estimation states for reservation of bufferresources.

BACKGROUND

In Long Term Evolution (LTE) networks, packet scheduling is modeled inthe Medium Access Control (MAC) layer and resides in the eNodeB (eNB). Ascheduler assigns radio resources, also called Resource Blocks (B), forthe downlink (termed assignments) as well as for the uplink (termedgrants) using the Physical Downlink Control Channel (PDCCH). The radiouplink is the transmission path from a terminal, which may also bereferred to as a User Equipment (UE), to a base station, or an eNodeB. Adownlink is the transmission path from the eNodeB to the terminal.

For uplink scheduling, the eNodeB requires information about the currentstate of the buffers in the terminal and in particular how much data theterminal has in its priority queues. This information is sent from theterminal to the eNodeB either as a 1-bit Scheduling Request (SR) or by aBuffer Status Report (BSR). The SRs are transmitted on a control channelsuch as e.g. Physical Uplink Control Channel (PUCCH) or Radio AccessChannel (RACH) while the BSRs are transmitted on the data channel suchas e.g. Physical Uplink Shared Channel (PUSCH), mostly together withuser data.

Precise and up-to-date scheduling information allows more accuratescheduling decisions, and can help to optimize the use and management ofradio resources and to improve network capacity. However, the accuracyof the information provided by the terminal is limited by thegranularity of the buffer status reports, by the frequency of the SR andbuffer status report transmissions and by the delay between thereception of the SR or buffer status report and the scheduling decision.For delay-sensitive services with periodical packet arrival, such asVoice over Internet Protocol (VoIP), the likelihood that the bufferstatus information is outdated when it is used is high. It is likelythat additional data has arrived since the buffer status report wastransmitted. It is also likely that the buffer will be emptiedfrequently and therefore the only available information will be a onebit SR.

With incorrect uplink information, the scheduler will provide either atoo large grant, which results in the terminal transmitting padding(essentially dummy data packets) and may reduce system capacity, or atoo small grant, which may lead to Radio Link Control (RLC) segmentationand increase transmission delay.

Uplink buffer status reports are needed in order for the base station toknow the amount of data waiting for transmission in the terminal. InE-UTRAN, uplink buffer status reports refer to the data that is bufferedfor a Logical Channel Group (LCG) in the terminal. Four LCGs and twoformats are used for reporting in uplink: A short BSR format containsthe buffer size of one LCG and a long BSR format contains the buffersizes of all four LCGs. Uplink BSRs are transmitted using MACsignalling.

According to the previously known solution in LTE, a framework forbuffer status reporting is specified. Buffer status reporting is used bythe terminal to report to the eNodeB the amount of data stored in itsbuffers for transmission. The eNodeB uses these reports to allocateresources to the terminal, and to prioritize resource allocation betweendifferent terminals. The terminal triggers a regular BSR and SR whenuplink data becomes available for transmission.

In the event that the terminal has data for more than one logicalchannel, the BSR can only contain information about one logical channel.A truncated format is available as padding Buffer Status Report. Anothertype of BSR Report, the Periodic BSR, provides a timer-based trigger perterminal to handle reporting for continuous flows.

Note that several logical channels can be mapped to a LCG so it is notpossible to determine the buffer status for one logical channel from aBSR unless the logical channel is the only logical channel in the LCG.An SR only indicates that the terminal requires resources for atransmission.

The packet sizes used by VoIP depend on several factors, such as RobustHeader Compression (used to reduce the packet size for VoIP), the IPversion (IPv4 and IPv6 have different packet sizes) and the VoIP codecused (which may provide high quality voice with a large packet size, orlower quality voice with a smaller packet size).

Scheduling of VoIP services in uplink may be made more efficient byestimating the buffer size. A VoIP connection is either proactive(speech frames are being transmitted) or passive (no speech frames arebeing transmitted, there is silence and Silence Indicator (SID) framesare transmitted.). When transmission of speech frames stops, thealgorithm moves from proactive to passive; when transmission of speechframes starts, the algorithm moves from passive to proactive.

One technique to determine when to move between states is based on thesize of received data packets, on the assumption that SID frames aresmaller than frames that contain voice data. This is described inWO2010090565A1. Another technique is based on analysis of the timebetween arrival of data packets. This is described in WO2012175113A1.

A problem with some of the known methods is that segmentation occurswhen the terminal has insufficient power to transmit a complete dataunit to the eNodeB with sufficient probability for successful detection.The data unit is segmented into two or more segments and the segmentsare transmitted in separate transmissions. Since the latency of thetransmission is one of the most important factors for the quality of aVoIP connection, the eNodeB will attempt to transmit the segments withina short time. During this time it is not possible to keep track of theamount of data in the terminal buffer by only using the received BSRbecause the buffer size as reported in the BSR is quantized, and the BSRwill often reach the eNodeB after the scheduling of the final segmentsof the data unit. The eNodeB is unable to sum the total amount of datareceived from the segments in time.

FIG. 1 shows a simplified picture of an uplink protocol stack withexamples of some vital functions for VoIP in each layer. A terminal 1communicates with an eNodeB via a physical layer 2. The MAC layer 3 isresponsible for functions such as like SR where a terminal 1 that doesnot have a grant can send a signal to the eNodeB indicating the need toget scheduled. The MAC layer 3 in the eNodeB is also responsible forreceiving BSRs and forward them to the scheduler. BSRs can only betransmitted when the terminal 1 has a grant and is triggeredperiodically (with a periodicity of for example 10 ms). Also, when a BSRis reported, logical channels are grouped into LCGs, where the logicalchannels within a group are supposed to have similar requirements.

For VoIP services, an RLC layer 4 provides multiplexing/de-multiplexingof logical channels. For a terminal using a VoIP service, the VoIPtraffic is typically assigned its own logical channel. For example, whenthe same terminal needs to send measurement reports, this is done usinganother logical channel. A PDCP layer 5 is also provided. The abovelayers are all eNodeB functionality. A VoIP client layer 6 is alsoprovided for decoding speech.

The known techniques for switching between the passive and proactivestates when estimating buffer size have several problems. For example,using the size of SID and TALK packets to determine when to switchstates is problematic because the size of those packets, as describedabove, depends on network configuration. It is therefore difficult toset reliable thresholds to determine when to switch between states. Thesize threshold may be between the size of a SID frame and the size of atalk frame.

Furthermore, the passive state may be entered if two or more consecutiveTBs containing only padding are received. WO2012175113A1 refers to usingthe time between receiving SRs to determine when to switch states.However, an SR does not distinguish between logical channels. It onlyindicates that the terminal has new data in one of its logical channels,and so is not suitable for estimating the time of arrival of VoIP datapackets.

A further problem occurs with segmentation of packets. When a BSR istriggered by the terminal, the number of bits that are currentlyavailable in the RLC buffer for each logical channel group aredetermined. This number is quantized to a 6 bit value (0 . . . 63)according to a table specified in 3GPP TS 36.321. This means that whenthe eNodeB receives a BSR it contains an overestimation of the buffersize, as the terminal needs to secure resources that are sufficient tocompletely empty its buffer. This over-estimation is normally not a bigproblem, but can become a problem when the state switching algorithmuses padding as a trigger for switching from the proactive to thepassive state. Consider the case where the terminal is in TALK state andthe channel quality is poor. In this case VoIP packets will besegmented, often into small transport blocks (less than 100 bits). Thescheduler will schedule the segments as often as possible, but thisscheduling is based on a BSR that over-estimates the amount of data inthe buffer. When the buffer is empty in the terminal, a padding BSR istriggered indicating that there is no more data to be scheduled, butthere can be several grants out-standing grants. These out-standinggrants will cause padding transmission by the terminal, and thesetransmissions can cause an erroneous switching from proactive to passivestate.

Finally, traffic on other logical channels can lead to incorrectswitching between states. Using BSRs for state estimation can causeconfusion when data arrives in the same LCG but for another logicalchannel than the VoIP data. For SRs, this problem is even more severe,since for these there is no indication of logical channel or logicalchannel group at all. 3GPP only provides 4 LCGs. It is not alwayspossible to reserve an LCG for the VoIP bearer. In many cases the VoIPbearer will not be the only bearer in the LCG. In case a video bearer ora bearer for VoIP control signaling is mapped to the same LCG as theVoIP bearer, it will be impossible to determine from a BSR if a terminalhas VoIP data in its buffer or other data. For example, when theterminal needs to transmit a measurement report, this is done using theRadio Resource Control (RRC) protocol, and an RRC message will be addedto the terminal RLC buffer. The terminal may trigger an SR due to this,and this SR may incorrectly trigger a switch from passive to proactivestate when received by the eNodeB.

SUMMARY

It is an object to obviate at least some of the disadvantages describedabove and provide an improved performance within a communication system.

According to a first aspect, the object is achieved by a method in anetwork node. The method provides a way to determine when to switchbetween buffer estimation states for a fixed frame rate services sessionat a terminal. In the event that the state is passive, the network nodedetermines a first time between two received Radio Link Control ServiceData Units (RLC SDU). If the first time is lower than a first timethreshold, then the state is changed to proactive. When the state isproactive, the network node determines that two consecutive paddingtransport blocks (TBs) have been received and further determines that asecond time between two padding transport blocks is greater than asecond time threshold. If so then the state is changed to passive.

As an option, the first time threshold is predetermined. This isoptionally fixed, or dynamically predetermined based on any of networkconditions and a codec used for the fixed frame rate services session,

The first time is optionally determined between two consecutivelyreceived RLC SDUs.

As an option, the second time threshold is predetermined. This isoptionally fixed or dynamically determined based on any of networkconditions and a known degree of RLC segmentation.

The second time is optionally determined between any of twoconsecutively received padding TBs and a time between first and lastreceived padding TBs.

The state is optionally changed from proactive to passive on the basison a number of padding transport blocks received.

An optional example of a network node implementing the method is a basestation.

As a further option, the method includes buffer estimation for theterminal based on the buffer estimation state.

The fixed frame rate services session is typically a Voice over IP(VoIP) session, but may be any other type of fixed frames rates servicessession such as a video session.

According to a second aspect, there is provided a network node for usein a wireless communication system. The network node is provided with aprocessor for determining when to switch between buffer estimationstates for a fixed frame rates service session at a terminal. A receiveris provided for receiving RLC SDUs, the processor being arranged to, inthe event that the state is passive, determine a first time between tworeceived RLC SDUs and in the event that the first time is lower than afirst time threshold, change the state to proactive. The processor isfurther arranged to, in the event that the state is proactive, determinethat two consecutive padding transport blocks have been received andfurther determine a second time between two received RLC SDUs and in theevent that the second time is higher than a second time threshold,change the state to passive.

As an option, the processor is arranged to use any of a predeterminedfirst time threshold and a predetermined second time threshold.

The processor is optionally further arranged to change the state fromthe proactive state to the passive state based on a size of a receivedRLC SDU being smaller than a size threshold.

The network node is optionally provided with a buffer, in which case theprocessor is arranged to estimate a buffer for the terminal based on thebuffer estimation state.

An optional example of the network node is a base station.

As an option, the fixed frames rates services session comprises any of aVoice over IP, VoIP, session and a video session.

According to a third aspect, there is provided a computer program,comprising computer readable code which, when run on a network nodecauses the network node to perform the method as described above in thefirst aspect.

According to a fourth aspect, there is provided a computer programproduct comprising a non-transitory computer readable medium and acomputer program as described above in the third aspect, wherein thecomputer program is stored on the non-transitory computer readablemedium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates schematically in a block diagram a simplified uplinkprotocol stack in LTE;

FIG. 2 illustrates schematically switching between passive and proactivestates;

FIG. 3 is a flow diagram showing exemplary steps of a method in a basestation;

FIG. 4 illustrates schematically exemplary switching between passive andproactive buffer estimation states; and

FIG. 5 illustrates schematically in a block diagram an exemplary networknode.

DETAILED DESCRIPTION

There is described herein a method and network node, which may be putinto practice in the embodiments described below. It will be appreciatedthat the network node may be embodied in many different forms and thefollowing description should not be constructed as limited to theembodiments set forth herein. These embodiments are provided so thatthis disclosure will be thorough and complete, and will fully convey thescope of the invention to those skilled in the art. It should beunderstood that there is no intent to limit the present methods and/orarrangements to any of the particular forms disclosed, but on thecontrary, the present methods and arrangements are to cover allmodifications, equivalents, and alternatives falling within the scope ofthe invention as defined by the claims.

The present embodiments are to be considered in all respects asillustrative and not restrictive, and all changes coming within themeaning and equivalency range of the appended claims are intended to beembraced therein.

In an embodiment, the network node described herein is a base station.It is to be understood that another configuration of base stationtransceivers may be connected through, for example, a mobile switchingcentre and other network nodes, to define a wireless communicationnetwork. The base station may be referred to as, for example, a RemoteRadio Unit, an access point, a Node B, an evolved Node B (eNodeB) and/ora base transceiver station, a Radio Base Station (RBS), an Access PointBase Station, base station router, etc. depending on the radio accesstechnology and terminology used.

In some embodiments, the terminal may be represented by a wirelesscommunication device, a wireless communication terminal, a mobilecellular telephone, a Personal Communications Systems terminal, aPersonal Digital Assistant (PDA), a laptop, a User Equipment (UE), acomputer or any other kind of device capable of managing radioresources.

The wireless communication network may be based on technologies such ase.g. Long Time Evolution (LTE), Global System for MobileTelecommunications (GSM), Enhanced Data rates for GSM Evolution (EDGE),General Packet Radio Service (GPRS), Code Division Multiple Access(CDMA), Wideband Code Division Multiple Access (WCDMA), CDMA 2000, HighSpeed Downlink Packet Data Access (HSDPA), High Speed Uplink Packet DataAccess (HSUPA), High Data Rate (HDR) High Speed Packet Data Access(HSPA), Universal Mobile Telecommunications System (UMTS) etc.

Further, as used herein, the wireless communication network may further,according to some embodiments, refer to Wireless Local Area Networks(WLAN), such as Wireless Fidelity (Wi-Fi) and Worldwide Interoperabilityfor Microwave Access (WiMAX), Bluetooth or according to any otherwireless communication technology.

The base station is adapted to schedule uplink transmissions from theterminal to the base station. In order to grant a particular terminalaccess to a particular uplink resource, a grant is sent from the basestation to the terminal, based on the estimated buffer status at theterminal. The expression “downlink” is here used to specify thetransmission from the base station to the terminal, while the expression“uplink” is used to denote the transmission from the terminal to thebase station.

The following description uses an exemplary VoIP session to illustratethe principles of estimating buffer size. This is by way of exampleonly. It will be appreciated that the same techniques may be used forany type of fixed frame rate services session including, but not limitedto, VoIP sessions and video sessions.

FIG. 2 illustrates the buffer estimation state, and the conditions underwhich a switch between the passive and the proactive state is made.

Incorrect estimation of the buffer state can lead to an inaccuratereservation of buffer resources at the base station for a VoIP sessionat the terminal. A Service Data Unit (SDU) is any kind of RLC SDU, andis transmitted on a VoIP logical channel. An SDU is therefore specificto a logical channel and can be positively associated with a VoIPsession. Basing switching on SDU reception can therefore remove theproblem of switching cause by packets sent on other, non-VoIP logicalchannels. It is proposed to switch the buffer estimation state from thepassive to the proactive state if a first time between reception of SDUs(which may be consecutive SDUs) is lower than a first time threshold.This is because TALK frames are typically sent more frequently than SIDframes. For example, a TALK SDU may be sent every 20 ms during theproactive state, whereas a SID SDU may be sent ever 160 ms. By settingthe first time threshold between those values, the buffer estimationstate can be accurately switched from the passive to the active state.

As discussed above, the problem of segmenting of VoIP packets can leadto incorrectly switching from the proactive to the passive state. Thiscan lead to an inaccurate estimation of the state of the terminalsession, and cause incorrect reservation of buffer resources at the basestation. It has been realised that changing the switching from theproactive to the passive state based on the time between reception ofconsecutive padding transport blocks (TBs) addresses this. The switchfrom the proactive state to the passive state is therefore only made ifthe arrival time between the TBs containing no payload data (indicatingpadding) is larger than a second time threshold. A padding TB isconsidered to be a TB that contains only padding instead of payload.

Turning to FIG. 3, the steps of an exemplary embodiment in which thebuffer estimation state is initially passive (SID frames are being sent)are shown. The following numbering corresponds to that of FIG. 3:

S1. The buffer estimation state is passive, as SID frames are being sentfrom the terminal to the base station.

S2. A determination is made of a first time between reception of RLCSDUs. These may be consecutively received SDUs.

S3. A determination is made to ascertain whether the time betweenreceiving RLC SDUs is below a first threshold. This threshold may bepredetermined and may be dynamically determined based on factors such asnetwork conditions or the codec used for the session. For example, for acodec with a TALK frame interval of 20 ms and a SID frame interval of160 ms, the threshold may be set at around 40-50 ms. A different codecmay use different intervals, in which case the threshold would bedifferent. If the time between receiving RLC SDUs is not below the firstthreshold, then the procedure reverts to step S2.

S4. As the first time is below the first threshold, the bufferestimation state is changed to proactive, and buffer resources may bereallocated to the terminal accordingly.

S5. The buffer estimation state is proactive, as TALK frames are beingsent from the terminal to the base station.

S6. A determination is made of a second time between reception ofpadding TBs. These may be consecutively received padding TBs. Where morethan two padding TBs are received, the second time may be the timebetween consecutive padding TBs or the time between the first and lastreceived padding TBs.

S7. A determination is made to ascertain whether the second time betweenreceiving padding TBs is above a second threshold. This threshold may bepredetermined and may be dynamically determined based on networkconditions. If the second time is below the second threshold, then theprocedure reverts to step S6.

S8. As the second time is above the second threshold, the bufferestimation state is changed to passive, and the procedure reverts tostep S1.

FIG. 4 shows an exemplary switch from passive (SID frames being sent) toproactive (TALK frames being sent). There is a 160 ms time gap betweenthe first two received RLC SDUs, and only a 20 ms time gap between thesecond and third SDU. As this time gap falls below a predeterminedthreshold, then the buffer estimation state is switched to proactive. Inthe example of the switch from proactive (TALK frames being sent) topassive (SID frames being sent), when it is determined that a timebetween padding TBs exceeds the second threshold, a switch fromproactive to passive is made.

FIG. 5 illustrates schematically in a block diagram an exemplary networknode such as a base station. In this example, the base station is aneNodeB 7. The eNodeB 7 is provided with a processor 8 for determiningwhen to switch between buffer estimation states. A buffer 9 is providedthat can have resources allocated to the terminal. A receiver 10 isprovided that receives RLC SDUs. As described above, the processor 8determines on the basis of a time between the RLC SDUs and apredetermined threshold whether to switch from the passive to theproactive state. The processor 8 is also arranged to, in the event thatthe state is proactive, determine the second time between two receivedpadding TBs and in the event that the second time is higher than thesecond time threshold, change the state to passive. The processor 8 isfurther arranged to allocate buffer resources to the terminal on thebasis of the buffer estimation state.

A non-transitory computer readable medium in the form of a memory 11 isprovided which may be used to store data such as predetermined timethresholds. It may also be used to store a computer program 12 which,when executed by the processor 8, causes the processor to implement thetechniques described above. Note that the computer program 12 may bestored on an external non-transitory computer readable medium in theform of a second memory, examples of which include compact disks andflash drives. The computer program may be transferred from the secondmemory 13 to the memory 11, or directly to the processor 8 forexecution.

The techniques described above address the problem of switching betweenthe passive and proactive buffer estimation states when the respectivepacket sizes are unknown. There are no conditions required that includethe size of received packets or the buffer size in the terminal. Insteadthe switching is done based on the time between received packets as wellas an indication that no payload data was received in a transport block(i.e. a padding packet has been received).

The techniques described above also address the issue of incorrectswitching from the proactive to the passive buffer estimation stateswhen VoIP packets require extensive segmentation, for example because ofdeteriorating network conditions or reduced terminal power. As describedabove, segmentation together with an over-estimation of the terminalbuffer status may lead to reception of a number of padding TBs whilestill in proactive state which, using prior art algorithms based on anumber of padding TBs received, can lead to incorrect switching to thepassive state. The techniques described above address this byadditionally considering the time between reception of padding transportblocks has been received. This is sufficiently sensitive to determinewhen padding transport blocks are received as a result of segmentation.The second threshold can be set high enough to prevent switching when anumber of padding transport blocks are received within the time it takesfor a new buffer status report to take effect in the scheduler(typically 4-6 ms).

The techniques described above address the issue of incorrectlyswitching from the passive to the proactive buffer estimation state inthe event that there is non-VoIP traffic on other logical channels.Switching from the passive to the proactive buffer estimation state isonly based on reception of RLC SDUs, which are associated with thelogical channel such as that used for VoIP. The technique may easily beextended to filter out RLC SDUs that belong to other logical channelsthan the logical channel that is configured for e.g. VoIP.

It will be appreciated by the person of skill in the art that variousmodifications may be made to the above described embodiments withoutdeparting from the scope of the present invention as defined in theappended claims.

The following abbreviations have been used in this description:

3rd Generation Partnership Project (3GPP)

BSR Buffer Status Report

CDMA Code Division Multiple Access

EDGE Enhanced Data rates for GSM Evolution

GPRS General Packet Radio Service

GSM Global System for Mobile Telecommunications

HDR High Data Rate

HSDPA High Speed Downlink Packet Data Access

HSPA High Speed Packet Data Access

HSUPA High Speed Uplink Packet Data Access

LCG Logical Channel Group

LTE Long Term Evolution

MAC Medium Access Control

PDA Personal Digital Assistant

PDCCH Physical Downlink Control Channel

PDCP Packet Data Convergence Protocol

PDU Protocol Data Unit

PHY Physical layer

PUCCH Physical Uplink Control Channel

PUSCH Physical Uplink Shared Channel

RACH Radio Access Channel

RB Resource Block

RLC Radio Link Control

RoHC Robust Header Compression

RRC Radio Resource Control

SABE Service Aware Buffer Estimation

SDU Service Data Unit

SID Silence Insertion Descriptor

SR Scheduling Request

TB Transport Block

UE User Equipment

UMTS Universal Mobile Telecommunications System

VoIP Voice over IP

WCDMA Wideband Code Division Multiple Access

WiFi Wireless Fidelity

WiMAX Worldwide Interoperability for Microwave Access

WLAN Wireless Local Area Network

The invention claimed is:
 1. A method in a network node of a wirelesscommunication system for determining when to switch between bufferestimation states for a fixed frame rate services session at a terminal,the method comprising, at the network node: in the event that the stateis passive, determining a first time between two received Radio LinkControl Service Data Units (RLC SDUs) and in the event that the firsttime is lower than a first time threshold, changing the state toproactive; and in the event that the state is proactive, determiningthat two consecutive padding transport blocks have been received,further determining whether a second time between two padding transportblocks is greater than a second time threshold, and in the event thesecond time is greater than the second time threshold, changing thestate to passive.
 2. The method according claim 1, wherein the firsttime threshold is predetermined.
 3. The method according to claim 2,wherein the first time threshold is based on any of network conditionsand a codec used for the fixed frame rate services session.
 4. Themethod according to claim 1, wherein the first time is determinedbetween two consecutively received RLC SDUs.
 5. The method according toclaim 1, wherein the second time threshold is predetermined.
 6. Themethod according to claim 5, wherein the second time threshold is basedon any of network conditions and a known degree of RLC segmentation. 7.The method according to claim 1, wherein the second time is determinedbetween any of two consecutively received padding transport blocks and atime between first and last received padding transport blocks.
 8. Themethod according to claim 1, further comprising changing the state fromproactive to passive on the basis on a number of padding transportblocks received.
 9. The method according to claim 1, wherein the networknode is a base station.
 10. The method according to claim 1, furthercomprising buffer estimation for the terminal based on the bufferestimation state.
 11. The method according to claim 1, wherein the fixedframes rates services session comprises any of a Voice over IP (VOIP)session and a video session.
 12. A network node for use in a wirelesscommunication system, the network node comprising: a processor arrangedfor determining when to switch between buffer estimation states for afixed frame rates service session at a terminal; a receiver configuredto receive Radio Link Control Service Data Units (RLC SDUs); theprocessor being arranged to, in the event that the state is passive,determine a first time between two received RLC SDUs and in the eventthat the first time is lower than a first time threshold, change thestate to proactive; and the processor being further arranged to, in theevent that the state is proactive, determine that two consecutivepadding transport blocks have been received and further determinewhether a second time between two padding transport blocks is greaterthan a second time threshold, and in the event the second time isgreater than the second time threshold, change the state to passive. 13.The network node according to claim 12, wherein the processor isarranged to use any of a predetermined first time threshold and apredetermined second time threshold.
 14. The network node according toclaim 12, wherein the processor is further arranged to change the statefrom the proactive state to the passive state based on a size of areceived RLC SDU being smaller than a size threshold.
 15. The networknode according to claim 12, further comprising a buffer, the processorarranged to estimate a buffer for the terminal based on the bufferestimation state.
 16. The network node according to claim 12, whereinthe network node is a base station.
 17. The network node according toclaim 12, wherein the fixed frames rates services session comprises anyof a Voice over IP (VOIP) session and a video session.
 18. The networknode according to claim 12, wherein the first time is determined betweentwo consecutively received RLC SDUs.
 19. The network node according toclaim 12, wherein the second time threshold is predetermined.
 20. Thenetwork node according to claim 19, wherein the second time threshold isbased on any of network conditions and a known degree of RLCsegmentation.
 21. The network node according to claim 12, wherein thesecond time is determined between any of two consecutively receivedpadding transport blocks and a time between first and last receivedpadding transport blocks.
 22. A non-transitory computer readable mediumcomprising computer program instructions which when executed by aprocessor, cause a network node to perform as follows: in the event thatthe state is passive, determining a first time between two receivedRadio Link Control Service Data Units (RLC SDUs) and in the event thatthe first time is lower than a first time threshold, changing the stateto proactive; and, in the event that the state is proactive, determiningthat two consecutive padding transport blocks have been received,further determining whether a second time between two padding transportblocks is greater than a second time threshold, and in the event thesecond time is greater than the second time threshold, changing thestate to passive.
 23. The non-transitory computer readable mediumaccording to claim 22, wherein the first time is determined between twoconsecutively received RLC SDUs.
 24. The non-transitory computerreadable medium according to claim 22, wherein the second time thresholdis predetermined.
 25. The non-transitory computer readable mediumaccording to claim 24, wherein the second time threshold is based on anyof network conditions and a known degree of RLC segmentation.
 26. Thenon-transitory computer readable medium according to claim 22, whereinthe second time is determined between any of two consecutively receivedpadding transport blocks and a time between first and last receivedpadding transport blocks.